Induction furnace



May 23, 1933. H.,w. PARKER 1 32 INDUCTION FURNACE Filed Juiy 9, 1950 1IINVENTIOR Henry W. Parker.

ATTORNEY Patented May 23, 1933 UNITED STATES PATENT OFFICE HENRY W.PARKER, OF TORONTO, ONTARIO, CANADA, ASSIGNOR TO ROGERS RADIO TUBES,LIMITED, 01 TORONTO, CANADA, A CORPORATION OF ONTARIO, CANADA INDUCTIONFURNACE Application filed July 9, 1930. Seria1 N0. 466,867.

charge devices such as thermionic tubes, an

induction furnace in the form of an inductance coil forming part of atuned circuit supplied with radio frequency current encloses the tubebeing exhausted and is used to heat, by induction, the metallic elementswithin the tube envelope to assist the process of removing the gaseswithin the tube.

The potential gradient across the ends of the inductance coil of such afurnace, when excited by radio frequency energy and the potentialgradient from the coil to ground, reach very high values and produce aglow discharge in the low pressure gases within the tube beingexhausted. Positively charged particles of gas moving under a voltagegradient have an equivalent kinetic energy temperature which isrelatively very high and thus cause electrochemical activity known inthe art as cleanup which prevents the gas from being conducted out ofthe tube by the pumps. This unduly prolongs the pumpin interval. Myinvention is directed towar preventing such electrochemical cleanupwhile the tube is being baked in the furnace and pumped and thus permitsof a larger amount of the gas being handled and removed during thepumping operation to reduce the time interval occupied by such pumping.

One of the objects of my invention consists in providing an improvedform of induction furnace, by means of which the time interval requiredfor pumping the tube is decreased.

Another object contemplated by my invention comprises providing means bywhich the aging process is greatly reduced intime.

A further object of my invention consists in providing a. furnace by theuse of which the final vacuum achieved in the tube is greatl increased.

Stil

another object contemplated by my invention is the production of ahigher and more uniform quality of product by the use thereof.

I accomplish all of the above and other desirable features which willhereinafter be pointed out and discussed, b the introduction of anelectrostatic shield between the inductance coil and the tube orotherobject being heated in the furnace, as shown in the drawing accompanyingand forming a part of the specification and in which:

Fig. 1 illustrates diagrammatically in section an elevation of myimproved furnace.

Fig. 2 is a perspective view of the electrostatic shield used.

Referring now particularly to the figures. My invention comprises aninductance coil 2, preferably constructed of hollow copper tubing,through which a cooling fluid is circulated and to which is connected avariable tuning condenser 8, terminals 9 and 10 being provided for thetuned circuit thus formed to which is attached a source of radiofrequency or other energizing high frequency current.

Within the coil or inductance 2 there is provided a cylinder 3 ofinsulating material and concentric therewith there is inserted anelectrostatic shield 4 between which and the tube or other object beingheated there is provided a further cylinder of insulating material 7.The electrostatic shield 4. is provided with an integral lug 5 affordinga means for grounding the shield as shown. The shield 4 comprises aslotted cylinder provided with teeth in the form of a comb as shown. Another accepted form of. electrostatic shiel may of course be employed,the object of the shield being to prevent electrostatic coupling betweenthe tube being baked and pumped and the inductance coil 2, and at thesame time to permit of electromagnetic coupling between the coil and theelements within the tube.

When electrostatic coupling between the coil and the tube is thusprevented the potential gradient of the radio frequency voltage acrossthe ends of the coil 2 and between the coil and ground is prevented fromexciting a glow discharge in the gases with in the tube and consequentlyelectrochemical cleanup is eliminated and it is'possible to reserve suchcleanup by a glow discharge to a later step in the process whichpreferably takes place after the pumping 15 completed and after the tubeis sealed off. This process is known to the art as aging and thefunction of this aging process is to produce an adsorption or cleanu ofthe gases by electrochemical means an at such time the low discharge isdesirable to rapidly prod lce a high vacuum after the pumping operationhas been completed.

By the use of an electrostatic shield such as just described, a largeramount of gas is removed from the tube by the pump during the intervalreserved for pumping the tube and after sealing off a relativelyuncontaminated adsorption surface is provided for electrochemical cleanuof the gas. I have found by experiment t at the aging process is reducedfrom one hour to two minutes by the use of an electrostatic shield andthat the final vacuum is thus increased tenfold. I have found that theroduct, that is, thermionic tubes, produced by the use of such a shieldare of a hi her quality and more uniform and that it is possible to morerapidly pump the tubes than hitherto.

While I have thus completely described one embodiment of my device, itis of course to be clearly understood that I may modify the detailsthereof without departing-from the spirit or narrowing the scope of myinvention.

Having thus described my invention, what I claim as new and desire tosecure by United States Letters Patent is as follows:

1. An inductance furnace comprising, a member formed of insulatingmaterial and open at both extremities thereof to receive an object to beheated, a single water-cooled inductance coil enclosing said member, andan electrostatic shield comprising a slotted member, the non-slottedrtion of which is maintained out of the eld of said induc- Y tanoe coildisposed between said inductance coil and the object to be heated andmain tained at ground potential.

2. An inductance furnace comprising, an open ended member of insulatingmaterial, an inductance coil formed of metallic tubing through which, acooling fluid is circulated disposed about one surface of said member,an electrostatic shield in the form of an open ended slotted metallic clinder, maintained at ground potential, the non-slotted portion of whichis maintained out of the eld of said inductance coil and disposed aboutthe op to surface of said member and a secon open ended member ofinsulating material disposed between said shield and the object to beheated.

HENRY W. PARKER.

